This invention relates to the metallurgical art and has particular relationship to high-strength, austenitic, non-magnetic alloys which are used in environments where they are subject to stress-corrosion cracking and/or to hydrogen embrittlement. Such alloys have general utility but they are uniquely suitable for use in the parts of large electrical generators (typically 1250 megawatt generators) and particularly for the end-winding retaining rings and the baffle rings of such generators. In the interest of facilitating the understanding of this invention, this application, in dealing with the use of the alloys, is confined to a specific concrete problem, namely, to such use in retaining rings and baffle rings of large generators. It is not intended that this treatment of the alloys in this application shall in any way restrict the scope of this invention. It is an object of this invention to provide wrought, austenitic, non-magnetic alloys, having general utility but being uniquely suitable for the above-mentioned parts of generators, which are characterized by a high rate of work hardening during cold working, i.e., characterized by a large increase in hardness or yield strength for a given degree of cold working, and also have high resistance to stress-corrosion cracking and hydrogen embrittlement.
A rotor of a large generator consists essentially of a single large forging, the main body of which contains a number of longitudinal slots which hold the copper conductors of the DC field winding. The conductors are retained in the slots by means of non-magnetic metal wedges anchored in grooves near the top of each slot. At the ends of the main body of the rotor the conductors emerge from the slots to join circumferential arc portions of the windings, thus forming a continuous series coil wound around the unslotted pole portions of the forging. That portion of the winding beyond each end of the forging body is called the end turn and must be retained against the centrifugal forces acting upon it up to speeds 20% above normal operating speeds (typically 3600 RPM) and higher. This retaining function is performed by the retaining ring. The ring rotates with the rotor and in addition to the load from the copper end turns to which it is subject, it is subject to an additional hoop stress which is proportional to the ring density and its mean radius. In fact, for steel alloys, about 68% of the ring stress is caused by the ring mass itself.
An essential feature of the rotor construction is that the ring is shrunk onto a fit on the rotor body at one end of the ring. The interference at the fit is sufficient to assure that looseness will not occur at 20% overspeed (4320 RPM for a rated 3600 RPM 2-pole machine). Insulation must be provided between the winding and the ring for voltages in the range 300-700V DC.
For many decades there has been continuous demand for increased ratings of turbine generators. This demand has necessitated corresponding increases in rotor diameters, to achieve these increased ratings without excessive rotor lengths. Increases in rotor diameters demand higher stresses in all rotating parts and higher strength materials are required. The highest stressed components of a rotor are the retaining rings.
The processing steps in the manufacture of a retaining ring involve electric furnace melting, sometimes electroslag remelting to get a cleaner ingot and a minimum of segregation, hot forging, hot piercing, hot expanding, solution treatment, quenching, cold expansion and stress relief anneal. The high yield strength of rings is obtained by cold expansion which may be accomplished by mechanical means with wedges, by hydraulic pressure, or by explosive forming. Sometimes, combinations of these techniques may be used. In the case of explosive forming, there is evidence that the intensity of shock wave loading should be minimized to avoid increasing susceptibility to stress-corrosion cracking.
Briefly, some of the desired characteristics of a retaining-ring material are the following: a high yield strength to avoid plastic deformation under high stress, a low density and high elastic modulus to minimixe deflection during overspinning, and a high thermal expansion coefficient to minimize the temperature required for the shrink fit (to avoid thermal damage to the electrical insulation).
Another desideratum is that the retaining rings be non-magnetic. The use of magnetic rings on a rotor results in greater magnetic end flux leakage with resulting extra heating in the stator coil ends and iron losses in the end region of the core. Additional excitation is required to compensate for this leakage and total machine efficiency is reduced.
The most pessimistic assumption on the exposure of a retaining ring to fatigue stresses is that the turbine-generator would be started and stopped once a day and subjected to a 10% overspeed test once a month during its lifetime. A 30 to 40 year life thus corresponds to a maximum of about 14,500 stress cycles. In the case of retaining rings, there is thus a low-cycle fatigue requirement.
Baffle rings are annular members approximately 2 in. square that are shrunk onto the rotor body at several positions along the length to channel the flow of the cooling gas. Baffle rings are made by the same process and from the same alloy as the retaining rings and have essentially the same property requirements.
Retaining and baffle rings in service in hydrogen-cooled generators are exposed to a pressure of from about 15 to 85 psig dry hydrogen gas, so that alloys for these applications should be resistant to static-load hydrogen-assisted crack propagation (hydrogen embrittlement). The case for requiring high resistance to stress-corrosion cracking is not as obvious, since the generator environment does not normally expose these materials to stress-corrosion conditions. However, a water leak in a foreign-built water-cooled generator recently caused stress-corrosion failure of a retaining ring having a composition in accordance with the teachings of the prior art.
Moreover, during steps in fabrication of rings or during storage or shipment there are numerous opportunities for accidental exposure to potentially corrosive environments, such as moist industrial or marine atmospheres, salt spray, welding flux fumes, fire extinguisher powders, liquid spills or leaks and snow or rain. The residual stresses from cold forming were sufficient to cause stress-corrosion cracking of some early retaining rings exposed to these conditions (Document 2). Even higher stresses are present after the ring is shrunk onto the rotor or from centrifugal forces when the generator is running. There have been several instances of retaining ring failures during generator operation that were attributed to stress-corrosion cracking (Documents 3 and 4).
The most searching method for evaluating the suitability of materials for service in a generator is by environmental testing of fracture toughness specimens. Fatigue precracked WOL (wedge-opening-loading) or CT (compact tension) specimens, preferably large enough to provide plane-strain loading conditions, are tested in various environment, such as salt water, H.sub.2 or H.sub.2 S, for static crack growth rate (da/dt) as a function of stress intensity for determination of K.sub.ISCC, K.sub.IH.sbsb.2, or K.sub.IH.sbsb.2 S, and fatigue crack growth rate (da/dN) as a function of .DELTA.K.
a is crack length. PA1 N is number of cycles of fatiguing. PA1 .DELTA.K is the stress intensity range used in fatiguing the specimen. PA1 (da/dN) is change in crack length per cycle of fatiguing. PA1 (da/dt) is change in crack length per unit time.
K.sub.ISCC is a threshold stress intensity, ksi .sqroot.in., below which a sharp crack will not grow under plane-strain conditions in a corrosive environment, such as salt water, hydrogen or hydrogen sulphide gas. K.sub.ISCC depends upon composition of the environment and temperature, pressure and time of exposure. K.sub.IH.sbsb.2 (apparent), for example, represents the stress intensity for crack propagation in 80 psig hydrogen gas at room temperature (70.degree. F) with a loading rate of 20 pounds/minute in a rising load test (performed with the apparatus shown in FIG. 4).
K.sub.IH.sbsb.2 S.
K.sub.Ic, the plane-strain fracture toughness, measures the resistance of a material to fracture in a neutral environment in the presence of a sharp crack under severe tensile constraint, such that the state of stress near the crack front approaches tritensile plane-strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions in the constraint direction. Calculation of K.sub.Ic is based on procedures established in American Society for Testing and Materials Standard E339-72.
There are many Cr-Mn-Ni-C-N-X steels in the prior art (X stands for one or more additional alloying elements, such as Mo, W, V, Cb, etc.). Although some of these steels may contain the same elements as are present in alloys according to this invention, they differ in quantity and proportion of alloying elements in one or more substantial ways from the alloy of this invention. The following Table I shows compositions of a number of these alloys, including several which have been used and have been proposed for use for retaining rings and baffle rings of large high power generators. The compositions of Table I are disclosed in the Related Documents above. The number in the third column of Table I is the number of the Related Document where the composition listed in the corresponding row is disclosed. By far, most of the items in Table I are not used or intended for retaining rings and baffle rings for large generators, but are actually used for entirely unrelated purposes, such as welding materials in the as-deposited condition or high-temperature alloys in the solution treated condition. Such alloys are not normally cold-worked. The numbers in the third column from the left in this table refer to items in "Reference to Related Documents".
Since it has been found that Cr is the most important element (although not the only one), in controlling stress-corrosion cracking of material that is rapidly cooled some prior art alloys are arranged in the order of increasing Cr contents in Table I for convenience of discussion. TBL3 TABLE I Prior Art Mn-Cr-Ni Alloys - Balance Essentially iron Proposed Designa- Ref.* By tion No. Cr Mn Ni C N Si Mo W V Cb Ta Ti Cu P B Other McCoy E9 8 0 16 .3 McCoy E5 8 0 20 .26 McCoy E3 8 0 25 .29 Abex 9 0 14 2 .45 2 .8 Baumel 6 64 .26 20.8 .1 .46 .002 2.03 Co Bungardt 10 3.9 9.2 8.4 .7 2.04 Manganello 11 4-5 1.75-19.5 .45-.6 .06-.12 .2-.5 Suzuki 12 4.7 18 1.9 .42 .01-.1 Kroneis 13 5 18 .36 .12 Speidel 5 5 18 .1 .5 Standard Steel 14 5 18 .5 .5-1.8 Japan Steel MV3 15 5 18 .5 3 .8 McCoy E7 8 5 15 .3 General Elec. 16 3.5-6 16.5-20.5 .4-.6 .25-1 Westinghouse 174-6 16-20 &lt;2 .4-.6 Opt. &lt;.5 &lt;.5 &lt;.2 Opt. Leitner 18 5-25 3-18 3-27 &lt;.3 .3-6 .3-6 .SIGMA. V,Ti,Ta,Zr,Co,Si&lt;3; Mo+W = .3-6 Cihal 17483 19 8.2 19.4 .13 .04 .37 .56 .49 Clarke 20 9-14 4-20 4-10 .1-.4 &lt;.3 0-3.5 0-3.5 0-.75 .15-.35 C+N &gt; .3 Dyrkacz 21 9-15 8-15 .6-1 .25-1.25 1.5-4 Heger 21 62 8.0 8.7 4.1 .38 .43 Heger 21 62 0-20 * 0-12 .25-1 1-4 .3-3 1.2-4 Al, *Sufficient Mn to form austenite Prause 365 63 8.0 23.9 - .02 .16 Japan Steel 15 10 18 .5 1.7 Japan Steel 15 10 18 .5 3 1.5 Schempp 22 10-30 .5-15 3-25 .2-.3 &lt;.4 &lt;3 &lt;3 &lt;3 .15-1 0-3 Mo+W; Ni + Mn = 12-30; C+P &gt; .45 Fleischmann 23 10-20 5-10 10-20 &lt;.1 .1-.2 .4 4-8 Norwood 24 10-30 .5-7 4-30 .01-.5 0-.2 .05-.25 10.times.C Bohler 25 10-23 4.7-9 5.5-10.2 .08-.2 .8-1.5 .01-.5 Cihal 17482 19 10.8 18.1 .10 .02 .5 .55 DeLong 26 11-20 10.5-19 0-4 .15-.5 0-.3 0-5 0-5 0-2 0-2 Mn + 2 Ni = 13-22 DeLong 27 11-21 9-19 0-4 .2-.6 0-.3 0-5 0-5 0-2 0-2 Mn + 2 Ni = 13-22 DeLong 28 11-21 9-19 0-4 .2-.85 0-.3 0-5 0-5 0-2 0-2 0-5 Mo+W; 0-2 V+Cb Drykacz 29 11.5-13.5 16-20 .2-.4 .1-.25 .15-.75 2-4 .6-.95 .1-.4 Clarke 30 11.5-15.5 0-16 0-8 0-.2 0-.2 0-0-3 0-1 1-5 0-.5 Ti, S, Se, Be Bohler 31 12 18 2.2 .06 1.05 .57 .6 Kohl 6 12 18 1.9 .15 .15 .5 Jennings 3212-30 7-20 .3-.6 .01-1 .3-.6 &lt;4 0-9 C + N &gt; .4 Hsiao 33 12-28 10-28 .15 .1-.8 .1-.8 .25 Jennings 34 12-30 3-12 2-35 .08-1.5 .06-.4 &lt;.45 Jennings 35 12-30 3-12 2-35 .08-1.5 &lt;.6 &lt;.45 1.5-9 Linnert 36 12-30 14.7-23.1 7-35 &lt;.08 &lt;3 0-4 0-1.5 0-1.5 0-5 Gimmill 37 12-18 3-10 6-10 .05-.25 .5-4 0-3.5 0-1.75 .25-2 Mo + W &lt; 4; Cb + V &lt; 2 Korchynsky 38 12-25 10-20 4-18 &lt;.6 .1-.6 2-6 1-4 Franks 3912-18 1-22 0-14 0-.1 .05-.18 Kroneis A6 13 13.5 19.5 .12 .25 Kroneis A7 13 14 25 .50 .25 Araki 40 14-22 4-13 5-18 .1-.4 &lt;.5 .5-4 1-4 &lt;4 &lt;2.5 Lutes 41 14.5 14 1 .35 .62 1.65 .62 .026 Kroneis B1 13 14.6 20.6 .53 .20 1.3 Furman 42 15-25 5-15 10-25 .3-.5 .05-.5 .9-1.5 .75-1.5 Whittenberger 43 15-21 12-18 0-3 .1 .25-.45 .5 Suzuki 12 15.6 20.7 .56 .25 .55 2 Becket 44 16-22 5-15 &lt;.3 Becket 45 16-22 5-15 &lt;.3 0-3 Becket 46 16-22 5-14 &lt;.12 .2-1 Becket 47 16-22 3-12 2-11 &lt;.3 Mn + Ni = 6-14 Becket 48 16-22 5-11 3-6 &lt;.15 .25-1.5 .25-2 Mn &gt; Ni; Mn + Ni &lt; 14 DeLong 49 16 16 1 .25-.45 0-4 0-4 0-2 0-1 Aborn 50 16 17 1 .15 Reidrich 68 51 16.6 12 1.2 &lt;.06 .2-.25 &lt;.2 .3- .5 Cihal 17460 19 17-20 7-10 4-6 &lt;.12 .12-.25 &lt;.9 Carney 52 17-18.5 14-20 .05-1 .06-.15 .25-1 .25-1 Gunsburg 53 17-18 2-8.7 2-6.3 .12-.4 .4-.65 Amer. Silver Magnil 54 17-19 14.5-16 &lt;.75 .08-.12 &gt;.35 .3-1 d'Imphy NMFX1 55 17.3 12 .12 .37 .27 Spaeder 56 18 15 5.5 .08 .4 .4 d'Imphy NMFX1 55 18 12 .2 .37 .4 Crucible Gaman R 57 18 12.5 .2 .35 .4 Benson 58 1815.9 5.5 .08 .4 .4 Franks 59 20-30 2-6 5-25 .01-.5 .01-.5 .5-3.5 Mo, Ti or Cb Armco 22-4-9 60 20-23 7-10 3-5 .45-.6 .3-.5 &lt;1 Payson 61 21-27 9-15 .55-.8 .3-.5 0-2.5 0-2 0-2 0-2 *See Reference to Related Documents.
The preferred prior art alloys for use for retaining rings and baffle rings have been steel alloys including, in weight percent, 18 manganese, 5 chromium and 0.5 carbon and, as shown in Table I, small quantities of other elements in addition to iron. As shown in Table I, there are many alloys for other purposes which contain in excess of 10% by weight chromium and also contain manganese in appreciable or substantial quantities.
The 18 Mn-5 C-0.5 C alloy has been cold worked to ever increasing yield strengths in attempts to meet the demands of increased rotor sizes. When environmental factors are considered, the strength limit for this alloy has essentially been reached. Further increases in rotor diameters will demand the use of retaining ring materials of higher strength than is afforded by the prior art alloys and with improved resistance to degradation in the service environment at these high strength levels.
This need for an improved alloy has been demonstrated by field experience and by studies which have been conducted. For example, M. O. Speidel recently used the fracture mechanics approach to evaluate the properties of an explosively formed 18 Mn-5 Cr-0.5 C retaining ring. At a yield strength of 174 ksi and with the excellent fracture toughness in air of 133 ksi .sqroot.in., the threshold stress intensity, K.sub.ISCC, for propagation of a crack in various aqueous solutions was only 6.4 ksi .sqroot.in. This would correspond to a critical flaw size below the limit of detection by the best ultrasonic inspection techniques, which means that undetected flaws could grow in the service environment to a size that would cause failure by the K.sub.Ic criterion.
Another limitation of the current 18 Mn-5 Cr-0.5 C alloy is that it readily becomes sensitized and this has an adverse effect on stress-corrosion cracking resistance. For example, Kohl (Document 6) has shown that sensitization, from inadvertent or deliberate aging in the temperature range of rapid carbide precipitation, can increase susceptibility to stress-corrosion cracking. Since retaining rings are massive forgings of thick cross section and low thermal conductivity, it is possible that carbide precipitation, principally at grain boundaries, could occur, especially in the midwall position in the ring, during cooling from the solution temperature through the critical temperature range of about 1400.degree.-1000.degree. F (760.degree.-538.degree. C) unless particular attention is paid to obtaining the best possible quench, as by using a large volume of cold quenching fluid with vigorous spray or agitation.
Under the most favorable quenching conditions, the cooling rate at the midwall position of a 5.7 in. thick ring of prior art alloy has been measured as 2.2.degree. F/sec (1.4.degree. C/sec). The cooling rate at the center of the retaining ring is important, as well as that at the surface, because, after being expanded as a simple hollow cylinder, machining of the end to shape exposes the interior of the ring to the environment. There is a small benefit in cooling because of heat extraction from the end during the quench, but the effect is not great 31/2 in. from the end. Moreover, material is frequently removed from the end of the ring for qualification mechanical tests, which would increase the effective quenching distance.
It is accordingly an object of this invention to surmount the difficulties and disadvantages of the prior art and to provide alloys which, while having general applicability, shall be uniquely suitable for retaining rings and baffle rings of large generators of ever increasing ratings. It is also an object of this invention to provide a generator whose retaining rings and baffle rings are composed of these alloys. It is also an object of this invention to provide a method for increasing the strength of these alloys.
Another object of this invention is to provide cold worked, austenitic, non-magnetic alloys that can be aged to increase hardness and yield strength and yet retain good resistance to stress-corrosion cracking and hydrogen embrittlement.
A further object of this invention is to provide an austenitic alloy composition that can be solution-treated and quenched in heavy sections up to about 4 to 6 in. thick and then be cold worked to a high-strength level and still be substantially non-magnetic and resistant to stress corrosion cracking and hydrogen embrittlement even when the interior of a heavy section, exposed by machining, is subsequently subjected to hostile environments during manufacture, storage or service.
It is also an object of this invention to provide alloys substantially less sensitive to stress corrosion cracking and hydrogen embrittlement than the prior art alloys of Table I.
Also, it is an object of this invention to provide manganese, chromium, carbon steel alloys having a yield strength of about 170 to 210 ksi, particularly for large electric generator parts, which alloys should be resistant to stress-corrosion cracking and hydrogen embrittlement.